Numerical dynamo models of Uranus' and Neptune's magnetic fields

The non-axisymmetric, non-dipolar magnetic fields of Uranus and Neptune are markedly different from the axially-dipolar dominated fields of the other planets in our Solar System with active dynamos. Stanley and Bloxham [Stanley, S., Bloxham, J., 2004. Nature 428, 151-153] used numerical modeling to demonstrate that Uranus' and Neptune's unusual fields could be the result of a different convective region geometry in these planets. They found that a numerical dynamo operating in a thin shell surrounding a stably-stratified fluid interior produces magnetic field morphologies similar to those of Uranus and Neptune. This geometry for the convective region was initially proposed by Hubbard et al. [Hubbard, W.B., Podolak, M., Stevenson, D.J., 1995. In: Cruickshank, D. (Ed.), Neptune and Triton. Univ. of Arizona Press, Tucson, pp. 109-138] to explain both the magnetic field morphology as well as the low intrinsic heat flows from these planets. Here we examine the influence of varying the stable layer radius in numerical models and compare the results to thin shell models surrounding solid inner cores. We find that a limited range of stable-layer shell thicknesses exist in which Uranus/Neptune-like field morphologies result. This allows us to put constraints on the size of the convective layers in Uranus and Neptune.     

Star Formation in NGC 4038/4039 as seen in the NIR

We obtained various sets of near infrared observations of the prototypical merger, NGC 4038/4039 (‘the Antennae’). Integral field spectroscopy and broad- and narrow band imaging aimed at obtaining age and extinction estimates of the young star clusters seen in large numbers distributed throughout the disks of the interacting galaxies. High resolution spectroscopy led to estimates of the dynamical masses of the clusters. The clusters have ages ranging from 3.7 to ≈ 20 Myrs. Those in the ‘overlap region’ are very young (below 8 Myrs), while in the nothwestern loop ages are above that limit, and the nuclear starbursts are much older (∼ 100 Myrs). Some photometric cluster masses lie above 10⁶ M_⊙. The stellar velocity dispersions determined from the medium- to high resolution spectra yielded virial cluster masses again up to a few 10⁶ M_⊙. Large differences in the estimated photometric and virial masses suggest a variation of the IMF between the clusters. At least some of the clusters have masses, concentrations and IMFs that could allow them to evolve into globular clusters. This revised version was published online in September 2006 with corrections to the Cover Date.     

Resonant interaction between two planetary waves as a precursor for stratospheric warmings?

Stratospheric warmings are attributed to an enhanced planetary wave activity, occurring nearly each winter – at least in the northern hemisphere – with different strengths. The generation of stratospheric warmings is not totally understood. One of the most promising explanations is the interaction of planetary waves: in many cases, the amplitude of the quasi-stationary planetary wave 1 builds up, until it transmits its momentum and energy to the background wind field. The role of wave 2 is usually considered to be less important.Based on ERA-40 and DYANA temperature data (January–February 1990), we found evidence that a resonant wave–wave interaction between a travelling and a stationary wave 2 was responsible for a minor stratospheric warming in February 1990. The interaction being observed during four weeks can eventually be used as an indication for an upcoming stratospheric warming.     

Paleomagnetism of the Cu�CZn�CPb-bearing Kupferschiefer black shale (Upper Permian) at Sangerhausen, Germany

Syngenetic, diagenetic and epigenetic models have been proposed for the Cu?CZn?CPb Kupferschiefer mineralization at Sangerhausen, Germany. Paleomagnetic and rock magnetic measurements have been made on 205 specimens from mine workings on the margin of the Sangerhausen Syncline. The mineralization is richest in the ??0.5-m-thick Upper Permian (258?±?2?Ma) Kupferschiefer black marly shale (nine sites) and dies out over ??0.2?m in the underlying Weisliegend sandstones (three sites) and overlying Zechstein carbonates (two sites). Except for one site of fault zone gypsum, characteristic remanent magnetization directions were isolated for all 14 sites using alternating field and thermal step demagnetization. These directions provide a negative fold test, indicating that the remanence postdates Jurassic fault block tilting. Rock magnetic measurements show that the Kupferschiefer shale marks a redox front between the oxidized Weissliegend sandstones and non-oxidized Zechstein carbonates. The 14 site directions give a Late Jurassic paleopole at 149?±?3?Ma. It is significantly different from the paleopole reported by E.C. Jowett and others for primary or early diagenetic Rote F?ule alteration that gives an age of 254?±?6?Ma on the current apparent polar wander path and is associated with Kupferschiefer mineralization. We suggest that the Late Jurassic extensional tectonic event that formed the nearby North German Basin also reactivated Variscan basement faults and extended them up through the overlying strata, thereby allowing hydrothermal basement fluids to ascend and epigenetically mineralize the Kupferschiefer shale. The possibility of a 53?±?3?Ma mineralization age is also considered.     

Petroleum source rocks of the Tarfaya Basin and adjacent areas, Morocco

This work characterizes the source rock potential of the Tarfaya Basin and enabled us to reconstruct its geochemical history. Outcrop samples covering different stratigraphic intervals, plus the northwestern part of the Zag/Tindouf Basin (Bas Draa area), were analyzed for total organic carbon (C_org) and total inorganic carbon contents and total sulfur content. Rock-Eval analysis and vitrinite reflectance measurements were performed on 56 samples chosen on the basis C_org content. A set of 45 samples were extracted and non-aromatic hydrocarbons were analyzed by way of gas chromatography-flame ionization detection (GC-FID) and GC-mass spectrometry (GC-MS). Non-isothermal open system pyrolysis at different heating rates was applied to obtain kinetic parameters for modelling petroleum generation from four different source rocks.High quality petroleum source rocks with high C_org content and hydrogen index (HI) values were found for samples of Eocene, Coniacian, Turonian and Cenomanian age. Most samples were carbonate rich and organic/sulfur values were high to moderate. Various maturity parameters indicated immature or possibly early mature organic matter. Based on organic geochemical and petrological data, the organic matter is of marine/aquatic origin (Cenomanian) or a mixture of aquatic and terrigenous material (Eocene). The Early Cretaceous interval did not contain high quality source rocks, but indications of petroleum impregnation were found.     

Phyllosilicates synthesis: a way of accessing edges contributions in NMR and FTIR spectroscopies. Example of synthetic talc

A series of new-generation synthetic talcs were prepared by varying the hydrothermal synthesis duration from a few hours up to 2 months. Crystallinity and particle size analysis of the synthetic products were evaluated by photon correlation spectroscopy, transmission electron microscopy, differential thermal analysis and unoriented X-ray diffraction, and then analyzed by various spectroscopic methods such as Fourier transformed infrared spectroscopy and solid-state ¹H and ²⁹Si magic angle spinning nuclear magnetic resonance. The new process for preparing synthetic talcs allows to obtain single-phased particles which after few hours grow heterogeneously and simultaneously in both the c* direction and the (ab) plan. Fine particles, observed for whatever the synthesis duration, lead to the presence of numerous sheet edges surfaces due to particle size which represents the main difference with natural talc. Spectroscopy data show the influence of the fine particles on signals and highlight that synthetic talc characterization is a potential tool for better understanding crystal chemistry of natural talc.     

Persistence of Artemisia steppe in the Tangra Yumco Basin,west-central Tibet,China: despite or in consequence of Holocene lake-level changes?

The closed Tangra Yumco Basin underwent the strongest Quaternary lake-level changes so far recorded on the Tibetan Plateau. It was hitherto unknown what effect this had on local Holocene vegetation development. A 3.6-m sediment core from a recessional lake terrace at 4,700 m a.s.l., 160 m above the present lake level of Tangra Yumco, was studied to reconstruct Holocene flooding phases (sedimentology and ostracod analyses), vegetation dynamics and human influence (palynology, charcoal and coprophilous fungi analyses). Peat at the base of the profile proves lake level was below 4,700 m a.s.l. during the Pleistocene/Holocene transition. A deep-lake phase started after 11 cal ka BP, but the ostracod record indicates the level was not higher than ~4,720 m a.s.l. (180 m above present) and decreased gradually after the early Holocene maximum. Additional sediment ages from the basin suggest recession of Tangra Yumco from the coring site after 2.6 cal ka BP, with a shallow local lake persisting at the site until ~1 cal ka BP. The final peat formation indicates drier conditions thereafter. Persistence of Artemisia steppe during the Holocene lake high-stand resembles palynological records from west Tibet that indicate early Holocene aridity, in spite of high lake levels that may have resulted from meltwater input. Yet pollen assemblages indicate humidity closer to that of present potential forest areas near Lhasa, with 500–600 mm annual precipitation. Thus, the early mid-Holocene humidity was sufficient to sustain at least juniper forest, but Artemisia dominance persisted as a consequence of a combination of environmental disturbances such as (1) strong early Holocene climate fluctuations, (2) inundation of habitats suitable for forest, (3) extensive water surfaces that served as barriers to terrestrial diaspore transport from refuge areas, (4) strong erosion that denuded the non-flooded upper slopes and (5) increasing human influence since the late glacial.     

New high‐resolution age data from the Ediacaran–Cambrian boundary indicate rapid,ecologically driven onset of the Cambrian explosion

The replacement of the late Precambrian Ediacaran biota by morphologically disparate animals at the beginning of the Phanerozoic was a key event in the history of life on Earth, the mechanisms and the time‐scales of which are not entirely understood. A composite section in Namibia providing biostratigraphic and chemostratigraphic data bracketed by radiometric dating constrains the Ediacaran–Cambrian boundary to 538.6–538.8 Ma, more than 2 Ma younger than previously assumed. The U–Pb‐CA‐ID TIMS zircon ages demonstrate an ultrashort time frame for the LAD of the Ediacaran biota to the FAD of a complex, burrowing Phanerozoic biota represented by trace fossils to a 410 ka time window of 538.99 ± 0.21 Ma to 538.58 ± 0.19 Ma. The extremely short duration of the faunal transition from Ediacaran to Cambrian biota within less than 410 ka supports models of ecological cascades that followed the evolutionary breakthrough of increased mobility at the beginning of the Phanerozoic.